Growing numbers of people are wirelessly accessing systems such as the internet and cellular telephone systems. This can lead to congestion in the limited spectral region designated for these wireless communication systems. As a result of this congestion, access to these systems can be denied or throughput over these crowded bandwidths can be reduced. Effectively, there is only a finite amount of space in these parts of the radio spectrum. As more devices go wireless, e.g., not just laptops or cell phones, but sensor networks, radio frequency ID tags, and other devices, all these devices also have to share the finite and increasingly crowded spectrum. One solution to increase the utility of the limited spectrum is to increase the spectral efficiency of these limited windows of spectrum. Such attempts have been effective, but have a finite point where further improvements in efficiency become overly difficult to implement.
The problem is not a lack of total spectrum but rather is due to the way that total spectrum is allocated and used. Wherein the use of selected portions of total available spectrum is separately licensed to different primary users (e.g., television broadcast regions, commercial radio broadcast regions, emergency channel regions, etc.), secondary users can have limited access to these additional spectral regions, creating crowding in the limited spectral regions available to large numbers of users (e.g., Wi-Fi spectral regions, wireless telephone spectral regions, cellular telephone spectral regions, among other wireless communication systems). Thus, another solution is to use wider spectral regions by sharing unused portions of spectrum, licensed to primary users, with secondary users for wireless communications.
The Federal Communications Commission (FCC) in the United States (and similar agencies in other parts of the world, e.g., Ofcom in the United Kingdom) license or allocate spectrum in various bands, for example, AM radio, VHF television, cellular phones, citizen's-band radio, pagers, Wi-Fi, Bluetooth, and Walkie-Talkies, among others. However, where these bands of frequency are underutilized, traffic from more congested spectral regions can be shifted to make more efficient use of them. The FCC has determined that as much as 70% of the allocated spectrum may be sitting idle at a given location and time, even though that portion of spectrum has been allocated for use by a primary user, for example, cellular network bands can be overloaded while amateur radio and paging frequencies can be vastly underutilized. Where a particular spectral region's use changes over time and location, it is necessary to determine the use of that frequency by a primary user at each location and at each time before allowing a secondary user to access the same spectral region to avoid interference with the primary user's enjoyment of that particular spectral region.
Systems allowing secondary users to utilize bands allocated to primary users, whenever it would not cause any significant interference, are known as Cognitive Radio (CR) systems. CR systems are beginning to be developed and deployed through such efforts as the FCC making special allowances so that new types of wireless networks can test CR systems on unused television channels, and the Institute of Electrical and Electronics Engineers (IEEE) beginning standardization of CR protocols. Large electronics manufacturers, such as Intel, have also begun discussions and R&D efforts on producing reconfigurable radio hardware to facilitate CR system deployment.
Cognitive radio systems can generally be divided into two types, 1) Full Cognitive Radio (a.k.a., “Mitola radio” or “software radios”) in which every possible parameter observable by a wireless node or network is taken into account, and 2) Spectrum Sensing Cognitive Radio in which typically only the radio frequency spectrum is considered. These two types can further be divided into licensed band (IEEE 802.22) and unlicensed band (IEEE 802.15) cognitive radios depending on their use of either licensed (privately allocated) spectrum or unlicensed (publicly allocated) spectrum, respectively.
One method of determining available spectrum is by employing an energy detector. The detection of available spectrum can facilitate potential use of that available spectrum by a secondary user. Cooperative spectrum sensing can be employed to determine available spectrum over larger areas by cooperatively detecting available spectrum across multiple energy detector locations and aggregating the results to facilitate determinations relevant to the area covered by the network of cooperating energy detectors. Generally, cooperative spectrum sensing is conducted through two successive stages: spectrum sensing (e.g., detecting signals transmitted from a primary user at each of a plurality of CRs) and decision reporting (e.g., transmitting decisions from the plurality CRs to a common receiver). The common receiver can then indicate that radios in the area covered by the plurality of energy detectors (e.g., CRs that comprise energy detectors) can employ the spectral region where it is not being occupied by the primary user.
Where cooperative spectrum sensing systems communicate over channels that are subject to interferences, conventional systems of cooperative spectrum sensing can be subject to missed detection and/or false reporting of spectral occupation. The various embodiments of the subject disclosure can provide avenues to improve reporting over “real world” communication channels to provide a more robust cooperative spectrum sensing environment. In contrast to convention systems, by employing space-time and/or space-frequency coding over communications channels, a virtual antenna can achieve transmit diversity. In further contrast to conventional systems, where an unreliable reporting channel can cause loss of sensing information from one or more nodes, relaying sensing information through other reporting components can maintain cooperative spectrum sensing through relay diversity. Moreover, relay diversity can further be made more robust by employing algebraic coding.